Hearing aid feedback cancellation systems (for reducing or cancelling acoustic feedback from an ‘external’ feedback path from output to input transducer of the hearing aid) according to the prior art may comprise an adaptive filter, which is controlled by a prediction error algorithm, e.g. an LMS (Least Means Squared) algorithm, in order to predict and cancel the part of the microphone signal that is caused by feedback from the receiver of the hearing aid. FIG. 1a illustrates an example of this. The adaptive filter (in FIG. 1 comprising a ‘Filter’ part end a prediction error ‘Algorithm’ part) is aimed at providing a good estimate of the ‘external’ feedback path from the DA to the AD. The prediction error algorithm uses a reference signal together with the microphone signal to find the setting of the adaptive filter that minimizes the prediction error when the reference signal is applied to the adaptive filter. The forward path (alternatively termed ‘signal path’) of the hearing aid comprises signal processing (‘HA-DSP’ in FIG. 1) to adjust the signal to the impaired hearing of the user.
In feedback cancellation systems, it may be desirable to add a probe signal to the output signal. This probe signal can be used as the reference signal to the algorithm, as shown in FIG. 1b (output of block PS), or it may be mixed with the ordinary output of the hearing aid to form the reference signal.
Prior art feedback cancellation systems comprising a probe or noise generator used in the feedback path are e.g. disclosed in U.S. Pat. No. 5,680,467, U.S. Pat. No. 5,016,280 and EP 1203510. WO 2004/105430 describes a method and apparatus for suppressing oscillation in a signal identified as or suspected of containing an oscillation due to feedback. The method involves converting the signal into frequency bands in the frequency domain, applying, for a selected period of time, a randomly changing phase to the signal in at least one of said frequency bands, and reconverting the converted signal into an output wave form signal. The method is “breaking the loop” by randomizing the phase.
Ideally, the probe signal should be un-correlated with the acoustic input signal, be inaudible and have as much energy as possible. White noise signals have been proposed in some prior art references, but the level of the noise then has to be low in order to remain inaudible. Lower levels of the reference signal will usually cause less accurate estimation of the feedback path, or slower adaptation of the system.